A fertile agricultural soil appears dark coloured at the surface as compared to soil one metre down. The reason for colour of top soil is

Options

A. more moisture
B. rich in organic matter
C. rich in iron, calcium and magnesium
D. recent formation

Explanation

To answer the question about why fertile agricultural soil appears dark colored at the surface compared to soil one meter down, we can break it down into the following steps:

### Step-by-Step Solution:

1. **Observation of Soil Layers**:
- Notice that the top layer of fertile agricultural soil is darker than the soil located one meter below the surface.

2. **Understanding Soil Composition**:
- The topsoil (the upper layer) is known to be rich in organic matter, which includes decomposed plant and animal materials.

3. **Definition of Detritus**:
- Detritus refers to organic matter that has decomposed, such as fallen leaves, twigs, and other plant debris, as well as dead organisms.

4. **Role of Decomposition**:
- Microorganisms, such as bacteria and fungi, play a crucial role in the decomposition of this organic matter. As they break down the detritus, they convert it into humus, which is dark in color.

5. **Color Change with Depth**:
- As we move deeper into the soil (like one meter down), the concentration of organic matter decreases, leading to a lighter color in the lower layers of soil.

6. **Conclusion**:
- The dark color of the topsoil is primarily due to the high concentration of organic matter and the presence of humus formed from the decomposition of detritus.

7. **Final Answer**:
- Therefore, the reason for the dark color of the topsoil is that it is rich in organic matter.